This invention relates to a data base management system, and more particularly to a method and a device for controlling data access.
Application programs executed in a computer are divided into a plurality of units and the execution of each unit is managed under the control of an operating system. A task represents a locus of control for the execution of a program unit called task unit.
As a basic technology for the effective management of the data which are stored in a computer and are utilized by multiple numbers of tasks, a concept called data base has been applied. The system for managing the data base is generally called Data Base Management System (hereinafter referred to as DBMS).
The purpose of data base technology is to centralize data files prepared for each application in each organization.
When data is managed with separate concentration from all task units, the user can maintain the data easier than before, as a result of the reasonable management. On the other hand, such management will tend to result in longer retrieval time. To improve this, various considerations are necessary when constructing the data base management system.
The physical form of data base is a group of data codes stored in a memory. Users do not directly manipulate the physical form of data but manipulate the abstract data model. The data model represents the user's view of what is the data base. Furthermore, data sub-language (DSL) or data manipulation language (DML) is available for the user to access to the data model. Data definition language (DDL) is provided for the users to build the data model. Among the conventional data models, hierarchical data model, the network-type data model and the relational data model has been widely known. The data model adopted in the embodiments of the present invention is the third one.
The data base is stored in a sub storage device such as a magnetic disk device. The minimum logical unit of the data model is called an item. The data model defines the logical relations between each item. The DBMS consists of two sections: the data base definition section in which the data model is defined and the respective items are assigned to be fitted into the data model and the operation section which treats retrieving, adding, deleting and updating of the data base.
In a computer system using a prior DBMS or in a prior data base machine, the data operation instruction is decoded when data manipulation instructions are fetched and executed. At this moment, the sub memory is accessed and the object data is transferred to the main memory to be used by the instruction.
Therefore, when an item in the data base is read out and processed in a computer provided with the prior DBMS, time, in the order of milliseconds, is taken to specify the object item and perform the operations (i.e. retrieval, addition, deletion and updating) in the DBMS operation field, even in the simplest operation such as GET one item (which is an operation of reading out data from the storage and loading it into a predetermined register). Because of this, the DBMS made by an ordinal method is hard to be applied to an application which requires high speed operation such as in the case of real time control, which requires the order of microseconds for the operation described above.